Anode for oxygen evolution

ABSTRACT

Novel electrode for oxygen evolution comprising an electroconductive base provided with an outer coating containing a mixed material of tantalum oxide and iridium oxide and preparation and use thereof.

United States Patent De Nora et al.

ANODE FOR OXYGEN EVOLUTION Inventors: Oronzio De Nora; Giuseppe Bianchi;Antonio Nidola; Giovanni Trisoglio, all of Milan, Italy Assignee:Electronor Corporation, Panama City Filed: May 17, 1973 Appl. No.:361,022

Foreign Application Priority Data References Cited UNITED STATES PATENTS10/1971 Dewitt 204/290 F 14 1 Apr. 15, 1975 3,632,498 1/1972 Beer204/290 F 3,711,385 1/1973 Beer..... 204/59 3,751,296 8/1973 Bccr117/230 FOREIGN PATENTS OR APPLICATIONS 1.147.442 4/1969 United Kingdom117 230 Primary Examine'r.1ohn H. Mack Assistant Examiner-AaronWeisstuch Attorney, Agent, or Firm-Hammond & Littell [57] ABSTRACT Novelelectrode for oxygen evolution comprising an electroconductive baseprovided with an outer coating containing a mixed material of tantalumoxide and iridium oxide and preparation and use thereof.

10 Claims, No Drawings- ANODE FOR OXYGEN EVOLUTION STATE OF THE ART Invarious electrochemical process such as, for example, in the productionof chlorine and other halogens, the production of chlorates, theelectrolysis of other salts which undergo decomposition underelectrolysis conditions and other electrolysis processes, it hasrecently become commercially possible to use dimensionally stableelectrodes in place of graphite. These dimensionally stable electrodesusually have a film forming valve metal base such as titanium, tantalum,zirconium, aluminum, niobium and tungsten, which has the capacity toconduct current in the cathodic direction and to resist the passage ofcurrent in the anodic direction and are sufficiently resistant to theelectrolyte and conditions used within an electrolytic cell, forexample, in the production of chlorine and caustic soda, to be used aselectrodes in electrolytic processes. In the anodic direction, however,the resistance of the valve metals to the passage of current goes uprapidly, due to the formation of an oxide layer thereon, so that it isno longer possible to conduct current to the electrolyte in anysubstantial amount without substantial increase in voltage which makescontinued use of uncoated valve metal electrodes in an electrolyticprocess uneconomical.

It is, therefore, customary to apply electrically conductiveelectrocatalytic coatings to these dimensionally stable valve metalelectrode bases. The electrode coatings must have the capacity tocontinue to conduct current to the electrolyte over long periods of timewithout becoming passivated, and in chlorine production, must have thecapacity to catalyze the formation of chlorine molecules from thechloride ions at an anode. They must be electroconductive andelectrocatalytic and must adhere firmly to the valve metal base overlong periods of time under cell operating conditions.

The commercially available coatings contain. a catalytic metal or oxidefrom the platinum group metals, i.e., platinum, palladium, iridium,ruthenium, rhodium, osmium and a binding or protective agent such astitanium, dioxide, tantalum pentoxide and other valve metal oxides insufficient amount to protect the platinum group metal or oxide frombeing removed from the electrode in the electrolysis process and to bindthe platinum group metal or oxide to the electrode base. The binding andprotective metal oxide is usually in excess of the platinum group metalor oxide. Anodes of this nature have been described in British Pat. No.1,231,280.

In anodes for the recovering of metals by electrowin' ning, a continualsource of difficulty has been the selection of a suitable material forthe anode. The requirements are insolubility, resistance to themechanical and chemical effects of oxygen liberated on its surface, lowoxygen overvoltage, and resistance to breakage in handling. Lead anodescontaining 6 to percent antimony have been used in' most plants. Suchanodes are attacked by chloride if present in the electrolyte. This isthe case at the huge plant at Chuquicamata, Chile, where it is necessaryto remove cupric chloride dissolved from the ore by passing the solutionover cement copper, reducing the cupric to insoluble cuprous chloride.At this plant there was also developed an anode of a copper-siliconalloy, called the Chilex anode, used in a portion of the tank-room. Ithas a longer life but raises the power consumption because of greaterresistance and greater oxygen overvoltage.

Attempts to use mixed oxide coatings such as RuO TiO for oxygenevolution have not been satisfactory in commercial use becausepassivation takes place after 200 to 1,000 hours of operation at acurrent density of 1.2 KA per m The use of a Ta O Ru0 mixed oxidecoating improves the electrocatalytic activity and the life of the anodesomewhat but not enough for commercial use. The use of a TiO -IrOcoating has lower electrocatalytic activity.

OBJECTS OF THE INVENTION It is an object of the invention to provide anovel anode for oxygen evolution having an outer coating containing amixed material containing tantalum oxide and iridium oxide.

It is an additional object of the invention to provide a novel electrodewith an outer coating of tantalum oxide and iridium oxide doped toimprove the catalytic activity for oxygen evolution.

It is another object of the invention to provide novel electrodes havinga coating of a mixed material of Ta O -IrO on a valve metal alloy basehaving improved mechanical stability.

It is a further object of the invention to provide a novel process forthe electrowinning of metals.

These and other objects and advantages of the invention will becomeobvious from the following detail description.

THE INVENTION The novel electrodes of the invention are comprised of anelectroconductive base provided with a coating over at least a portionof its outer surface of a mixed material of tantalum oxide and iridiumoxide. The coating may be as little as 5% of the outer surface of theelectrode but preferably covers 50 to of the active face of theelectrode. The preferred ratio of tantalum to iridium calculated inpercent of metal is 1:1 to 0.34:1.

The electrode base may be made of any electroconductive material such asiron, nickel, lead, copper, etc. or alloys thereof but is preferably avalve metal such as tungsten, titanium, tantalum, niobium, aluminum orzirconium or alloys of two or more of said metals. The valve metalsbases may be provided with an intermediate layer such as an oxide of thevalve metal or a coating of another metal such as platinum group metals.The base may be a valve metal and at least either one metal having a lowhydrogen overvoltage such as alloy of titanium with iron, cobalt,nickel, palladium, vandadium or molybdenum, or mixtures of two or moreof said metals; or one metal suitable to form with titanium a protectiveoxide film even in acid solution such as an alloy of titanium withniobium, tantalum, zirconium or mixtures of two or more of said metals.

In a preferred embodiment of the invention, the electroconductive baseis an alloy of a valve metal with a platinum group metal which has animproved corrosion resistance to acid electrolytes encountered in theuse of the electrodes such as 5 to 15% sulfuric acid or 1 to 5%hydrochloric acid. A particularly useful alloy is titanium containing0.1 to 0.20% by weight of palladium. This corrosion resistance of thesupport of the coating prevents chipping off of the coating even if theanode is immersed for a few hours in an acid electrolyte without anodicpolarization.

In a modification of the invention, the coating containing tantalumoxide and iridium oxide can be doped with an oxide of a metal with avalence of less than +4 to increase the catalytic activity for oxygenevolution without adversely effecting the mechanical properties of thecoatings.

Without wishing to be limited to the following theoretical discussion,it is believed that the semiconductivity of the Ta O -IrO- system is ofthe n type and that the addition of the doping metal oxide reverses thetype of conductivity from n-type to p-type which improves the anodicprocess by producing electronic holes.

The doping metal oxide may be present in the coating in amounts rangingfrom 0.5 to 5.0% preferably 1.5 to 3.0% by weight of the said systemcalculated as metal. Examples of suitable doping metal oxides arealkaline earth metals such as calcium, magnesium, barium and members ofGroups VIII, VI B and VII B of the periodic Table such as cobalt, ironand nickel, chromium, molybdenum, manganese, etc.

The increase in the catalytic activity of the doped coatings is shown bythe lower anode potential of doped anodes as compared to undoped anodesafter 8,000 hours of operation of the anodes under identical workingconditions. The doping seems to have no adverse effect on the mechanicalproperties of the coatings as there is no coating loss in eitherinstance even after 8,000 hours operation.

The electrodes of the invention are particularly useful for electrolyticprocesses such as cathodic protection, electroflotation, organicelectrosynthesis such as hydrodimerization of acrylonitrile and mostparticularly the electrowinning of metals. The said electrodes have ahigh electrocatalytic activity and a very low passivation rate of a fewmillivolts per month at a current density of 1.2 to 2.0 KA per m and anegligible weight loss if kept under anodic polarization.

The novel method of the invention for the preparation of the electrodesof the invention comprises applying to an electroconductive electrodebase a solution of a thermally decomposible compound of tantalum and athermally decomposible compound ofiridium, drying the coated electrodebase by evaporation of the solvent and then heating the dried electrodebase in the presence of an oxygen containing gas such as to form thedesired electrode.

The heating step is preferably effected at temperatures of 350 to 600C,the optinum temperature being 500 to 550C. At temperatures below 350C,the oxidation is not completed or requires too long heating time and attemperatures above 600C, the electrode base is likely to be subjected todistortions and/or destruction by the high temperatures.

The preliminary drying step is preferably effected by gentle heating inair to evaporate the solvent and codeposit the metal compounds. However,any convenient procedure may be used to remove the solvent such asstanding under reduced pressure.

In a preferred embodiment of the process, the coating is applied inmultiple coats with short periods of intermediate heating such as 500 to550C for 5 to minutes with a longer final heating after the last coatsuch as 500 to 550C for 45 minutes to l 1% hours. The

coating obtained thereby is very adherent and quite uniform.

The electrodes of this invention are particularly useful forelectrowinning process used in the production of various metals becausethey do not add impurities to the bath which deposit on the cathode,.with the metals being won, and thereby contaminate the refined metal,as do anodes of for example lead containing antimony and bismuth whichgive impure cathode refined metals. Moreover, their resistance to theacid solutions and oxygen evolution and their excellent anode potentialmakes them desirable for this use.

In the following examples there are described several preferredembodiments to illustrate the invention. However, it should beunderstood that the invention is not intended to be limited to thespecific embodiments.

EXAMPLE I 24 Titanium plates 10 mm by 10 mm were etched in boiling 20%hydrochoric acid for 60 minutes and were then thoroughly washed withwater. The plates were then coated with an aqueous solution of thecompositions of Table I in 12 to 15 coats. After the application of eachcoat, the plates were dried and then heated for 10 minutes at 450C to600C in an oven with forced air circulation and then allowed to aircool. After the last coat, the plates were heated in the oven at thesame temperature for 1 hour and were then air cooled. The values ofTable I are calculated as weight of free metal. The tantalum chloridewas used as a solution in 20% hydrochloric acid.

The anode potential for each anode was then determined by electrolysisof 10% by weight sulfuric acid at 60C and a current density of 1.2 KA/mThe initial anode potential (against NHE) and the anode potential after3,000 and 6,000 hours was determined and the coating loss' was thendetermined. The values are reported in Table II.

TABLE IV Anode potential (NHE) in Volts after coating loss initial 600 hi000 h 1200 h in mg/cm anode potential and the anode potential after600, 1,000 or 1,200 hours are reported in Table IV. The final loss ofthe coating was determined at the end of 5 the test.

60C at a current density of 1.2 KA/m The initial Sample TABLE II Coatingweight Anode Potential V(NHE) loss initial after after value 3000 hs.6000 hs. mgjcni Sample Final heating in C The results of Table IV showthat RuO -TiO coated electrodes become passivated after only 1,000 hoursEXAMPLE Ill 10 plates of titanium containing 0.l5% of palladium 10 X 10mm) were sandblasted and then etched in refluxing hydrochloric acid for60 minutes. The

TABLE V Coating compositions in mg of Sample No. free metal plates werethen coated with the compositions of Table V. The compositions wereapplied in 15 to 20 coats 2 and the Ta O -RuO coated electrodes are onlyslightly improved and the TiO -lrO coated electrodes are no better.

with intermediate heating at 450 C for 10 minutes In an oven with forcedair circulation and cooling in air. The final heating was effected atthe temperatures in Table V for 1 hour followed by air cooling.

EXAMPLE II For comparative purposes, electrodes were prepared asfollows. Titanium plates 10 mm by 10 mm were The results of Table 11show that the electrodes of the invention have high electrocatalyticactivity and a very low passivation rate and that the weight loss of thecoating is negligible when within the limits of the invention. It shouldbe noted that the ratio of Ta to lr for samples F to F is about 034.Optimum values are obtained in the heating range of 500550C.

etched in boiling 20% hydrochloric acid for minutes and were thenthoroughly washed with water. The

OOOOOOOOWO 05050505 5 5555555555 0 0 m nmmmaeem s. r nmnmnmnmnm AB DEFH1 ABmDE mH 0 5 4 4 plates were then coated with an aqueous solution ofthe compositions of Table III in 12 to 15 coats. After the applicationof each coat, the plates were dried and then heated for 10 minutes at450 to 550C in an oven with forced air circulation and then allowed toair cool. After the last coat, the plates were heated in the ovenCoating weight loss in mg/cm after 2000 hs.

TABLE VI after The anode potentials and coating weight loss were sodetermined as in Example 1 and the results are reported Anode Potentialin V(NHE) initial value The results of Table VI show that the electrodesof the invention with a titanium palladium alloy base in Table VI.

Sample No.

heating temp in "C 5 .0 6O69696O6Q6 TABLE Ill Coating composition SampleNo. in mg (metal) The anode potential for each anode was then determinedby electrolysis of 10% by weight sulfuric acid at at the sametemperature for 1 hour and where then air cooled. The values of TableIII are calculated as weight of free metal.

have excellent electrocatalytic activity and low passivation rates.

EXAMPLE IV To demonstrate the improved corrosion resistance of atitanium palladium alloy, 10 plates made of titanium containing 0.15% byweight of palladium (10 X 10 mm) were sand-blasted and then etched inrefluxing 20% hydrochloric acid for 60 minutes. The plates were thencoated with the compositions of Table V using the procedure of Example111. The anode potential was determined for each electrode byelectrolysis of 10% sulfuric acid at 60C and a current density of 1.2KA/m The initial anode potential and the value after 1,000 and 2,000hours and the coating weight loss after 2,000 hours was determined.Moreover, the current was halted for minutes in each 24 hour periodwithout removing the electrode from the acid bath. The results arereported in Table V11.

The results of Table V11 show that the electrodes of the inventionhaving a titanium palladium alloy base have excellent electrocatalyticactivity and low passivation rates and the coating does not chip offeven without anodic polarization.

EXAMPLE V 10 titanium plates X 20 mm) were etched in refluxing 20%hydrochloric acid for 60 minutes and after being thoroughly washed withwater, the plates were coated with an aqueous solution containing 2.01mg (as free metal) of TaCl 3.2 mg (as free metal) oflrC1 and 0.0394 mlof hydrochloric acid. The solution was applied in 12 coats-withintermediate heating and cooling and a final heating as described inExample I.

The coated titanium plates were used as anodes in cells for the recoveryof zinc from an aqueous electrolyte containing 100 g/liter of Zn S0 (asfree metal), 10% sulfuric acid and 10 to 50 ppm of glue. The cathode wasa pure aluminum sheet with a smooth surface and the electrolyte gap was10 mm. The current density was 500 A/m and the electrolyte temperaturewas 35C. The anode potential, loss of coating, zinc thickness on thecathode and the morphology of the zinc deposit are reported in TableV111 The cathodic current efficiency was found to be 92-95% in all casesand the purity of the zinc deposit was 99.9999%.

EXAMPLE V1 Using the procedure of Example V, five titanium plates (20 X20 cm) were coated with the composition of Example V. The coated plateswere used as anodes in ace for recovery of copper from an aqueouselectrolyte containing 100 g/liter (as free metal) of CuSO and 10g/liter of sulfuric acid and the cathode was a smooth steel plate. Theelectrolyte gap was 15 mm and the bath temperature was C. The currentdensity was 500 A/m The anode potential, loss of coating and copperthickness and morphology of the copper deposit are reported in Table 1X.

TABLE 1X Test Anode potential Coating Cu deposit Cu deposit No. V (NHE)weight loss thickness morphology in mm 1 1.47 0 4. smoo The cathodiccurrent efficiency was found to be in all cases and the purity of thecopper was 99.9999%.'

EXAMPLE V11 TABLE x Specimen Liquid Coating per gm of noble metal/m eachtitanium sheet coupon Specimen Liquid Coating per gm of noble metal/mNo. each titanium sheet coupon 3.2 do. do. 1r l6 C.1Cl. ..6H ,O 0.21 do.do. C11

0.0374 mls. 4.4A.4B.4C Tt1Cl,-, 1.87 mg. Tu

lrCl 3.2 do. do. lr 1(1 CaCl2.6HgO 0.26 do. do. Ca HCl 0.0362 mls.

The samples were then tested in 10% sulfuric acid at TABLEXIII-Continued 2 60 C w an anodlc current. density of i KA/m to Speci-Temp. Co content Anode Potential Coating -determ1ne the anode potentialand coatmg loss after men fi l heat initial ft weight 2,500 hours. Theresults are shown in Table XI. treatmen value 8000 hs. loss 15 V(NHE)mglcm TABLE X1 18 550C do. 1.52 1.57 do. 1C do. do. 1.52 1.57 do. 2 500C2.5 1.52 1.53 do. Specimen Temp. Ca Anode Potential We1ght 2A do. do.1.52 1.53 do. No. final heat content initial after loss 28 550C do. 1.521.54 do. treatment b.w.t. value 2500 hs. mg/cm 2() 2C do. do. 1.52 1.54do. V(NHE) 3 500C 4 1.52 1.56 do. 3A do. do. 1.52 1.56 do. 5 a .5 L 5 03B 550C do. 1.52 1.56 do. 1,, C 1 1 t 2 do 3C do. do. 152 1.56 do. 1; 4500C 5 1.52 1.56 do. 1B 550 C l 1.51 1.56 do.

4A do. do. 1.52 1.57 do. lC do. 1.51 V 1.56 do. 7 a 4B 550 C do. 1.521.56 do. 500 C 2 5 1.50 1.51 do. 4C d d 1 l 57 d 2A do. 1.51 1.51 do.213 550C 2 5 1.50 1.52 do. I I 2C do. 1.50 1.52 do. Variousmod1ficat1ons of the electrodes are processes 3 500C 4 0 of theinvention may be made without departing from 3A do. 1.52 1.58 do. 3B550C 4.0 155 the sp1r1t or scope thereof and it should be understood 2C3106C 5 O g that the invention is to be limited only as defined in theO. 4A do. 1.52 1.60 do. appended, clams- 4B 550C 5.0 1.52 1.65 do. Weclaim: 4C 1. An electrode comprising an electroconductive base providedwith a coating over at least a portion of its outer surface of a mixedmaterial of tantalum oxide EXAMPLE V111 Using the procedure of ExampleV11, 20 X 20mm titanium coupons were coated with the composition ofTable X11 with the same heatings.

The anode potentials and the coating losses after 8,000 hours in 10%sulfuric acid at 60 C with an anodic current density of 1.2 KA/m wasdetermined as in Example V11 and the results are reported in Table X111.

TABLE X111 Speci- Temp. Co content Anode Potential Coating men finalheat 7: b.w.t. initial after weight treatment value 8000 hs. loss V(NHE)mg/cm l 500C 1 1.52 1.56 0 1A do. do. 1.52 1.56 do.

and iridium oxide, in which the ratio of tantalum to iridium calculatedas metal is 1:1 to 0.34 to l.

2. The electrode of claim 1 wherein the said base is a valve metal.

3. The electrode of claim 1 wherein the said base is an alloy of a valvemetal and at least one of the platinum group metals.

4. The electrode of claim 2 wherein the valve metal is titanium.

5. The electrode of claim 1 wherein the said base is an alloy containingat least two valve metals.

6. The electrode of claim 1 wherein the said base is an alloy containingat least two valve metals and at least 0.34 to l.

9. An electrode comprising an electroconductive base provided with acoating over at least a portion of its outer surface of a mixed materialof tantalum oxide and iridium oxide, in which the ratio of the tantalumto the iridium calculated as metal is 1:1 to 0.34 to 1, said coatingfurther contains 0.1 to 5.0% by weight of an oxide of a metal selectedfrom the group consisting of alkaline earth metals, cobalt, iron,nickel, chromium, molybdenum and manganese.

10. An electrode of claim 9 wherein the metal is selected from the groupconsisting of cobalt and an alkaline earth metal.

1. AN ELECTRODE COMPRISING AN ELECTROCONDUCTIVE BASE PROVIDED WITH ACOATING OVER AT LEAST A PORTION OF ITS OUTER SURFACE OF A MIXED MATERIALOF TANTALUM OXIDE AND IRIDIUM OXIDE, IN WHICH THE RATIO OF TANTALUM TOIRIDIUM CALCULATED AS METAL IS 1:1 TO 0.34 TO
 1. 2. The electrode ofclaim 1 wherein the said base is a valve metal.
 3. The electrode ofclaim 1 wherein the said base is an alloy of a valve metal and at leastone of the platinum group metals.
 4. The electrode of claim 2 whereinthe valve metal is titanium.
 5. The electrode of claim 1 wherein thesaid base is an alloy containing at least two valve metals.
 6. Theelectrode of claim 1 wherein the said base is an alloy containing atleast two valve metals and at least one of the platinum group metals. 7.The electrode of claim 1 wherein the base is an alloy of titaniumcontaining up to 0.2% by weight of palladium.
 8. An electrode comprisinga base of titanium alloyed with up to 0.2% by weight of palladiumprovided with a coating over 50 to 100% of the base of a mixed materialof tantalum oxide and iridium oxide, the ratio of tantalum to iridiumcalculated as metal being 1:1 to 0.34 to
 1. 9. An electrode comprisingan electroconductive base provided with a coating over at least aportion of its outer surface of a mixed material of tantalum oxide andiridium oxide, in which the ratio of the tantalum to the iridiumcalculated as metal is 1:1 to 0.34 to 1, said coating further contains0.1 to 5.0% by weight of an oxide of a metal selected from the groupconsisting of alkaline earth metals, cobalt, iron, nickel, chromium,molybdenum and manganese.
 10. An electrode of claim 9 wherein the metalis selected from the group consisting of cobalt and an alkaline earthmetal.